Lime mud is the calcium carbonate mud which is produced during the causticizing of the green liquor with quicklime in connection with chemical recovery in sulphate pulp mills.
The quicklime required for causticizing is for the main part obtained by reburning the lime mud, usually in a rotary kiln.
The rotary kiln is slightly inclined so that the lime mud which is fed in at the upper end of the kiln moves, as a consequence of the rotation of the kiln, towards the lower end of the kiln and meets the combustion gases from an oil, gas or solid fuel burner in the lower end of the kiln and is therely converted to quicklime. This quicklime is fed out from the lower end of the kiln.
The lime mud is fed into the rotary kiln with a dry solids content of 60-75%. The kiln is usually divided into three zones: a drying zone, an intermediate zone and a combustion zone. The drying zone, which normally comprises about one-third of the length of the kiln is, for most of its length, furnished with slack hanging chains having both their ends fastened to the kiln wall. The purpose of these chains is to stir the mud in order to obtain rapid and effective drying of the mud. The intermediate zone, which comprises about one-half of the length of the kiln, is normally furnished, wholly or in part, with longitudinal fins projecting from the kiln wall for stirring the fine granular material so as to improve the transfer of heat. The combustion zone, where the calcination of the calcium carbonate to calcium oxide takes place at a temperature in excess of 900.degree. C. and usually 1000.degree.-1400.degree. C., is lined with high-grade refractory insulating material. The combustion gases leave the rotary kiln with a temperature which is normally between 170.degree. C. and 240.degree. C.
Measurements during conventional operation of lime mud reburning kilns have shown a very much higher temperature for the outgoing combustion gases than that on the kiln wall. From this the conclusion can be drawn that a large part of the combustion gases pass through the center of the kiln with limited transfer of heat to the lime mud and that the heat economy of the process is poor.
It has earlier been known for measures to be taken to improve the heat economy of rotary kilns in connection with the burning of cement and similar materials. In German Pat. No. 618085 and Russian Pat. No. 830094 there is shown examples of such techniques. According to these patents measures have been concentrated solely on the chain zone, i.e., the zone in which the feed-in occurs and shortly thereafter. According to the German patent this zone is furnished with two or more stationary disks located at distance from each other and having central apertures and with baffles hung in chains and with free room in the radial direction around the baffles. At the same time the chain zone is furnished with slack hanging chains running in an axial direction. Additionally there is a shovel-like pusher plate on the inner surface of the kiln shell adjacent to the end of the feed-in pipe. The combustion gases are forced to move sinuously through the alternately located central apertures in the disks and the peripheral openings around the baffles. At the same time the chains pull through the material on the bottom of the kiln, dragging it up along the shell wall from where it falls down and comes into contact with the hot gases. The material also receives heat from the chains, which during their rotation absorb heat from the combustion gases. The construction is heavy and costly and its object is principally to achieve effective drying of the wet material fed in.
According to the Russian patent, the cold zone of the kiln, the feed section, is furnished with centrally suspended baffles placed near each other. The first of these baffles is conical and the subsequent baffles are conically shaped rings, which all lie relatively close to one another, even so close as to partially overlap one another. The maximum distance between the cones corresponds to 0.2 times the larger diameter of the preceding cone. A set of chains is arranged so that each chain hangs down over one of the cone-shaped surfaces and thereby frees that surface from dust. Another set of chains lies behind the baffles viewed in the direction of motion of the combustion gases. These latter chains have as their purpose to free the combustion gases from dust. A principal object of the invention in the Russian patent is to achieve constancy of flow velocity both along the length of the kiln and across its cross-section in order thereby to attain the best possible removal of dust from the gases and good transfer of heat in the feed section.